High-altitude pulmonary edema: the intercellular network hypothesis

The pathophysiology of high-altitude pulmonary edema is currently attributed to exacerbated heterogeneous hypoxic pulmonary vasoconstriction. However, although other cellular mechanisms have been hypothesized, they are still poorly understood. In this review, we focused on cells of the pulmonary acinus, the distal unit for gas exchange, known to be responders to acute hypoxia, notably through many humoral or tissue factors that connect this intercellular network constituting the alveolo-capillary barrier. Hypoxia could drive alveolar edema by: 1) damaging the fluid reabsorption capacity of alveolar epithelial cells, 2) increasing the endothelial and epithelial permeability, especially by alteration of occluding junctions, 3) triggering the inflammation mainly led by alveolar macrophages, 4) increasing interstitial water accumulation by disruption of extracellular matrix architecture and tight junctions, 5) inducing pulmonary vasoconstriction through an orchestrated response of pulmonary arterial endothelial and smooth muscle cells. Hypoxia may also alter the function of fibroblasts and pericytes that contribute to the interconnection of the cells of the alveolar-capillary barrier. Due to its complex intercellular network and delicate pressure gradient equilibrium, the alveolar-capillary barrier is simultaneously affected by acute hypoxia in all its components, leading to rapid accumulation of water in the alveoli.